[9.03] The unusual dynamics of new dark spots on Neptune.

Voyager 2 imaging of Neptune in 1989 revealed an earth-sized
southern Great Dark Spot (GDS) with an associated bright
poleward companion cloud. Since 1994 two new dark spots have
been observed in the northern hemisphere. One near
32\circ N was seen first in 1994 (Hammel et al.
1995, Icarus \bf 265, 1740) and last in 1996
(Sromovsky et al. 2000a Icarus, submitted),
always with a bright poleward companion. The second was seen
in 1996, with no companion, at a surprisingly low latitude
of 15\circ N (Sromovsky et al. 2000a).

While the 1989 GDS drifted equatorward, as expected from
simulations of anticyclones in a background anticyclonic
shear (e.g. Lebeau and Dowling 1998, Icarus \bf 132,
239), neither of the northern spots has exhibited
latitudinal drift. A possible explanation is that there are
local variations in zonal wind or temperature structure that
modifies the potential vorticity gradient in a way that
provides stability against drift. New circulation
measurements based on 1998 HST observations (Sromovsky
et al. 2000b Icarus, submitted), combined with
earlier wind measurements from 1995 and 1996, do show
regions of negative absolute vorticity gradients, but not at
latitudes that would explain the lack of dark spot drift.

The drift of the 1989 GDS toward the equator likely led to
its dissipation before the fall of 1990. The new dark spots
seem to have found some other mechanism for decay. Between
1996 and 1998 the two new dark spots both disappeared from
view (Sromovsky et al. 2000b). However, because the
companion clouds are still prominent in 1998, we believe
that NGDS-32 still exists as an anticyclone, even though not
visible as a dark feature in the F467M (blue-filter) images.
The bright cloud seen at ~40\circ N in July 2000
IRTF images of Neptune suggests that NGDS-32 might still
exist.

This work was supported by STScI grants GO-07324.01 (LAS),
GO-07324.02 (TED), and GO-7324.03(KHB), and NASA Grant
NAG5-6788 (LAS), and is based on observations with the
NASA/ESA Hubble Space Telescope, obtained at the STScI,
which is operated by AURA under NASA contract NAS5-26555.